Targeted disruption of the estrogen receptor-alpha gene in female mice: Characterization of ovarian responses and phenotype in the adult

Targeted disruption of the mouse estrogen receptor-alpha gene (estrogen rec eptor-alpha knockout; ERKO) results in a highly novel ovarian phenotype in the adult. The ERKO mouse model was used to characterize ER alpha-dependent processes in the ovary. Visualization of the ovaries of 10-, 20-, and 50-d ay-old wild-type (WT) and ERKO mice showed that the ERKO phenotype develope d between 20 and 50 days of age. Developmental progression through the prim ordial, primary, and antral follicle stages appeared normal, but functional maturation of preovulatory follicles was arrested resulting in atresia or in anovulatory follicles, which in many cases formed large, hemorrhagic cys ts. Corpora lutea were absent, which also indicates that the normal biochem ical and mechanical processes that accomplish ovulation were compromised. Northern and ribonuclease protection analyses indicated that ERKO ovary FSH receptor (FSHR) messenger RNA (mRNA) expression was approximately 4-fold g reater than in WT controls. Ovarian LH receptor (LHR) mRNA expression was a lso higher in the ERKO animals. Cellular localization studies by in situ hy bridization analysis of ERKO ovaries showed a high level of LHR mRNA expres sion in the granulosa and thecal layers of virtually all the antral follicl es. Ribonuclease protection analyses showed that ovarian progesterone recep tor and androgen receptor mRNA expression were similar in the two groups. T hese results indicated that ER alpha action was not a prerequisite for LHR mRNA expression by thecal or granulosa cells or for ovarian expression of p rogesterone receptor mRNA. Ovarian estrogen receptor beta (ERP) was detected immunohistochemically, wa s sharply compartmentalized to the granulosa cells, and was expressed appro ximately equally in the ERKO animals and the WT controls. In contrast, ER a lpha staining was present in the thecal cells but not the granulosa cells o f the WT animals. The summary findings indicate that in the adult the major cause of the ERKO phenotype is high circulating LH interacting with functional LHR of the th eca and granulosa cells. These features result in a failure of the normal m aturational events leading to successful ovulation and luteinization and pr esumably involve both hypothalamic-pituitary and intraovarian mechanisms de pendent upon ER alpha action. The presence of ER beta in the granulosa cell s did not rescue the phenotype of the ovary.